The appearance of two Mikoyan MiG-31 fighter interceptors in a group of six Russian Air Force (VVS) military aircraft intercepted by Canadian and US air force aircraft on 17-18 September suggests a change in Russia's approach to offensive operations.

According to US officials, the aircraft included the two MiG-31s, two Tupolev Tu-95 strategic bombers, and two Ilyushin Il-78 tankers - a modified version of the Il-76 military transport. The intercepts were the latest in about 50 such incidents over the past five years.

While fairly common during the Cold War, VVS exercises and simulated attack runs against North America dropped off with the collapse of the USSR. They have picked up again in recent years as relations between Washington and Moscow deteriorated.

The first intercept was at 1900 local time on 17 September by two US Air Force (USAF) Lockheed Martin F-22A Raptors operating about 55 n miles from the Alaskan coast. The second, at 0130 hours on 18 September, was by two Royal Canadian Air Force (RCAF) McDonnell-Douglas CF-18 fighters about 40 n miles from the Canadian coast in the Beaufort Sea.

Both intercepts were well inside the 200-mile air defence identification zone, although in neither case did the Russian aircraft enter US or Canadian airspace.

A North American Aerospace Defence Command (NORAD) spokesman, Lieutenant Colonel Michael Jazdyk, told reporters that the aircraft were scrambled "basically to let those [Russian] aircraft know that we see them, and in case of a threat, to let them know we are there to protect our sovereign airspace".

The MiG-31 was designed and built during the Soviet era almost exclusively as a defensive weapon system at a time when there were two separate air forces operated by the USSR military: the VVS, and the air defence forces (PVO). Both services operated fighter aircraft, but whereas the VVS mission was to support offensive operations of conventional ground forces and nuclear bomber assets, the PVO was to defend the USSR's air space and intercept incoming enemy aircraft.

The PVO's requirements made it a higher priority than that of the VVS, and the air defence service operated what were considered to be more sophisticated aircraft. One of the most famous USSR defectors, Lieutenant Viktor Belenko, who flew his MiG-25 to Hokkaido in Japan, was a PVO pilot and not from a front-line VVS squadron.

Both the MiG-31 and its predecessor, the MiG-25, were designed exclusively for the PVO mission and were not a fit with the VVS's requirements. "These were two different rival services," explained a veteran Russian aircraft designer. "Mikoyan was engaged to design the MiG-31 for the PVO because the air defence forces did not want to just be satisfied with purchasing aircraft designed for the VVS - in this time frame the newest and most modern VVS aeroplane was the Sukhoi Su-27 - in the same way that the US Navy do not ever want to procure fighters that were designed for the USAF."

When the two services were merged in 1998, the VSS found itself in control of assets that it was unfamiliar with and required a review of the post-Cold War security environment to determine how the new combined services roles and missions might need to change.

This resulted in modifications to the aircraft - the MiG-31BM variant - that included a modernisation of its NIIP N007 Zaslon radar and avionics configuration and the addition of an air-to-air refuelling probe that extends its internal fuel range from 1,450 km to 5,400 km.

The most significant change was the integration of a new missile that is a derivative of the Vympel R-33 (AA-9) air-to-air missile, most recently designated the RVV-BD (Missile Air-to-Air - Long Range).

The RVV-BD was originally intended to take out NATO battle management assets, such as the USAF Boeing E-3 AWACS. It now appears to be envisioned as being employed to intercept incoming cruise missiles or to equip fighters that escort bomber missions flying over the polar regions to attack targets in North America.

Developing a new mission for aircraft such as the MiG-31 took some time "because during Soviet times the two services of the VVS and PVO never conducted joint exercises," said the Russian aircraft designer, "but this incident shows that it has now been fully adapted for an offensive mission."